1. Field of the Invention
This invention relates to a high consistency heat-curable fluorosilicone rubber composition and a fluorosilicone rubber obtained by curing said composition. The heat-curable fluorosilicone rubber composition is cured into a cured product with optical clarity, and can be used for many diverse applications, for example, tubes, gaskets, diaphragms and the like.
2. Description of the Prior Art
The prior art teaches many ideas that relate to silicone rubber compositions with optical clarity. These silicone rubber compositions can be used for many diverse applications requiring optical clarity, for example, tubes, hoses, diaphragms, packings, contact lenses, etc.
It is necessary to blend an extrusion compatible crosslinker into silicon rubber compositions prior to the extrusion process and the subsequent curing process. Dichlorobenzoyl peroxide is a well-known extrusion compatible crosslinker which is widely used in such silicone rubber compositions. It is an unstable peroxide in the compositions and initiates a free radical reaction that forms crosslinkages between adjacent polymer chains. However, dichlorobenzoyl peroxide has a disadvantage of needing to go through the post cure process to remove harmful decompositional by-products.
As compared with the above peroxide curing compositions, addition catalyzed compositions do not require the time consuming post cure process but typically do to obtain maximum physical characteristics. The prior art teaches many ideas that relate specifically to high consistency silicone rubber compositions which are catalyzed by hydrosilation reactions and cured into silicone rubbers with optical clarity.
Addition reaction occurs between hydrosilyl group and vinyl groups, usually catalyzed by a platinum compound such as chloroplatinic acid, forming Si--C bonds as described in the following formula. ##STR2## This type of crosslinking system has some advantages over other kinds of vulcanizing agents. Once the proper components are allowed to react together, the composition can be vulcanized rapidly at elevated temperatures or much more slowly at room temperature.
Transparent silicone rubber compositions which can be vulcanize at room temperature are proposed by Hittamair et al (U.S. Pat. No. 3,408,325 and U.S. Pat. No. 3,474,064.)
The prior arts also teach many techniques which greatly enhance the optical clarity of silicone rubbers. One prior art technique of obtaining compositions possessing improved optical clarity uses matching the refractive index of the filler with that of the polydiorganosiloxane by utilizing one or more polydiorganosiloxanes which contain phenyl groups. U.S. Pat. Nos. 3,996,189 and 3,996,187, both to Travnicek, teach that 80 to 95 parts of one or two polydiorganosiloxanes containing from 6 to 16 mole percent phenyl groups can produce a material with adequate optical clarity when mixed with 5 to 20 parts of fumed silica. The same patents teach that the absence of phenyl groups incorporated into a polydiorganosiloxane results in hazy material. Hartlage, in U.S. Pat. No. 3,624,023, also teaches about transparent silicone rubber compositions which are vulcanizable under ambient conditions. This particular patent utilizes a surfacetreated fumed silica filler and a hydroxyl-end blocked polydiorganosiloxane which contains phenyl units.
Polmanteer et. al. in U.S. Pat. No. 4,418,165 teaches optically clear silicone rubber compositions comprising 100 parts by weight of at least one polydiorganosiloxane and 15 to 120 parts by weight of a hydrophobic reinforcing silica filler. It further states specifically that the refractive index of the polydiorganosiloxane does not have to be matched with that of the silica filler to obtain optically clear materials. The compositions of this invention can utilize polydimethylsiloxanes and poly-3,3,3-trifluoropropylmethylsiloxanes as long as the polydiorganosiloxanes chosen can be vulcanized to produce transparent compositions in the absence of silica fillers. The optical clarity is obtained through the use of a hydrophobic reinforcing silica filler consisting of surface-treated particles consisting essentially of SiO.sub.4/2 units wherein substantially all of the particles are of such a size that when properly incorporated into the polydiorganosiloxane, they do not appreciably scatter light and hence result in optically clear compositions regardless of the refractive index of the polydiorganosiloxane employed.